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Initial mutational steps toward loss of opsin gene function in cavefish
S Yokoyama1, A Meany, H Wilkens
1Department of Biology, Syracuse University, NY 13244-1220, USA.
Molecular Biology and Evolution
|July 1, 1995
Summary
Opsin gene analysis in Astyanax fasciatus reveals high mutation rates in eyed Micos fish and deletions in blind Pachon cavefish. These genetic changes suggest rapid evolution even before genes become nonfunctional.
Area of Science:
- Evolutionary Biology
- Genetics
- Ichthyology
Background:
- Astyanax fasciatus, a species of freshwater fish, exhibits significant adaptive radiation into cave environments.
- Ocular and anophthalmic (blind) populations of A. fasciatus provide a unique model for studying evolutionary changes in sensory systems.
- Opsin genes are crucial for visual pigment function and are expected to undergo selection in environments with varying light levels.
Purpose of the Study:
- To investigate genetic differences in opsin genes between surface-dwelling and cave-dwelling populations of Astyanax fasciatus.
- To identify specific mutations and sequence variations in red (r007) and green (g103) opsin genes.
- To understand the potential impact of these genetic changes on visual function and gene evolution.
Main Methods:
- Collection of A. fasciatus fish from riverine (Micos) and cave (Pachon) environments in Mexico.
- Sequencing of red (r007) and green (g103) opsin genes from three distinct populations.
- Comparative sequence analysis to identify nucleotide substitutions, transitions, and deletions.
Main Results:
- A high frequency of cytosine to thymine (C-->T) transitions was observed in the r007 and g103 opsin genes of the eyed Micos population.
- These C-->T changes are hypothesized to result from spontaneous cytosine methylation or enzymatic deamination of CG dinucleotides.
- A deletion of 12 consecutive nucleotides was identified in the g101 opsin gene of the blind Pachon population.
Conclusions:
- Elevated rates of nucleotide substitution and C-->T transitions, typically associated with pseudogenes, can occur in functional genes prior to their inactivation.
- The observed genetic variations in opsin genes highlight rapid evolutionary adaptation in response to different environmental pressures (presence or absence of light).
- This study provides insights into the mechanisms and rates of gene evolution in natural populations, particularly in the context of sensory system adaptation.